In recent years, advances in lithography techniques have lead to ongoing, rapid miniaturization of resist patterns. Recently, levels of resolution capable of forming line and space patterns of no more than 100 nm, and isolated patterns of no more than 70 nm, are being demanded.
As the microprocessing technology used for realizing these high resolution levels, exposure methods that use electron beam irradiation have been attracting considerable attention. Positive photoresist compositions that have been proposed as ideal photoresist materials for these methods that include an electron beam exposure step typically employ a polyhydroxystyrene-based resin, in which a portion of the hydroxyl groups have been protected with acid dissociable, dissolution inhibiting groups, as the base resin. Examples of the most commonly used acid dissociable, dissolution inhibiting groups include so-called acetal groups, including chain-like ether groups typified by 1-ethoxyethyl groups, and cyclic ether groups typified by tetrahydropyranyl groups, as well as tertiary alkyl groups typified by tert-butyl groups, and tertiary alkoxycarbonyl groups typified by tert-butoxycarbonyl groups.
(Patent Reference 1)
Japanese Unexamined Patent Application, First Publication No. 2000-227658
However, in microprocessing technology, the formation of high aspect ratio, fine patterns, and particularly line and space patterns of no more than 100 nm, has proven very difficult due to pattern collapse caused by the surface tension of the developing solution. One possible countermeasure involves reducing the thickness of the photoresist film, but if the film thickness is reduced using conventional photoresist compositions, then the etching resistance tends to be inadequate.